A PROCESS-MODEL OF PRECIPITATION ISOTOPES AND LEAF WAX δ2H ACROSS THE TIBETAN PLATEAU AND SOUTH ASIA

The δ2H and δ18O isotopic composition of ancient precipitation is crucial for studying the evolution of climate and topography over time. Usually, these isotopes are measured in a proxy, such as clay in a paleosol, hydrated volcanic glass, or hydrogen in leaf wax. A fractionation factor must be estimated to convert the proxy measurement back to the isotopic composition of the local meteoric water. We focus here on estimating the fractionation relationship between the δ2H of local meteoric water with the δ2H of leaf wax. The fractionation factor is thought to be affected by differences in vegetation and associated biosynthetic processes, but this effect is difficult to test. The reason is that leaf wax δ2H is set by the long-term average δ2H composition of the local meteoric water, but it is difficult to measure the local isotopic composition of meteoric water at a suitably long time scale (1 to 2 years and longer) and to a suitable precision.

We address this problem using a process-based program called OPI (Orographic Precipitation Isotopes) to estimate the time-averaged spatial distribution of meteoric water δ2H across the high Himalayas and Tibet at a grid resolution of 1 km. The mean result for the local precipitation isotope field is fit, using least-squares, to δ2H and δ18O of ~700 samples of modern base-flow river water from the region. The data are well fit by the model, and the resulting predicted δ2H values are estimated to have a standard error of 0.9 per mil. We then use this grid to interpolate meteoric water δ2H values at 200 locations where modern leaf wax δ2H has been measured. These data indicate that the δ2H fractionation between primary meteoric water (minimal evaporation) and leaf wax (n-C29 alkane) equals -115.8 per mil. This apparent fractionation factor is constant at about +/- 1 per mil across the entire Tibetan plateau and its margins. The regional consistency of this estimate demonstrates that the mean state for the δ2H of meteoric water is very steady, despite the observation of large variations on short-term time scales (1 day to 1 month). Our OPI-based method provides a way to estimate fractionation factors for other proxies and in other settings.